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P.S. Found out last night that the power amp I am currently using that I was given by a friend uses TL431 for the output stage bias. Which seems an eminently sensible use for it

We discussed this possible application in another thread. Delivering a constant voltage irrespective of temperature may be admirable for a voltage reference, but for biasing up a power amp output stage the commonly used Vbe multiplier (one transistor and two resistors) will exhibit a similar voltage shift with changing temperature as the output power devices, desirable to prevent thermal runaway. These Vbe multiplier bias transistors are routinely thermally bonded to the same heat sink as the power devices so they will track.

I don't doubt somebody stuck a TL431 inside a power amp output stage, I've seen worse things. Driving the output stage with a constant bias voltage, means the class A current will increase as the power devices heat up and their Vbe gets smaller, increasing the class A current even more and increasing the heat dissipation, dropping the Vbe even more... you can see where this is going. This can realize thermal equilibrium without releasing smoke "if" A) the emitter degeneration resistors are large enough to limit the class A current to some tolerable power dissipation, and/or B) the heat sink moves enough heat out to prevent too much temperature rise and Vbe drop. I recall seeing equations to quantify this (decades ago) but some of the equation terms are hard to put a finger on (like thermal resistance to ambient).

The TL431 IIRC is already borderline too much voltage for typical bias applications but perhaps the low voltage version (1.25V?) could use a temperature dependent resistor in the voltage setting feedback string to move the bias voltage in the correct direction with temperature change.

When I was product manager over all power amps for Peavey I inherited a value 2kW class A/B amp. Since this design scrimped on power devices to reduce cost it was borderline stable thermally and the service repair record reflected that. I added a pair of metal TO-3 devices per channel to lower the thermal resistance (junction to sink, and ultimately to ambient) to stabilize the amp. Adding several dollars to the BOM of a value amplifier was not embraced by Peavey management, but it was the right thing to do. These days a 2kW class A/B amp is a relic of times past.

At Peavey a widely used approach for the output stage bias string involved using a specific dual diode with a tightly specified temperature/voltage characteristic that was similar to the metal TO-3s used in large numbers. The dual diode was routinely inserted through a hole drilled/punched in the aluminum heatsink L-bar mounted on top of the PCB, and soldered in place.

JR

PS: I did a great deal of bench work last century designing a bias circuit that targeted regulating the actual class A current ignoring temperature. I had a working circuit but the engineering manager I reported to at the time lacked the cohones to let me use it in a production SKU . I still regret his bad judgement. (Another reason I promised myself to never be an employee again if i can possibly help it). I built my prototype bias circuit into a cheap guitar amp using plastic darlington power devices that was tragically under biased when cold... the stock amp would literally sound like crap until it warmed up and realized decent class a bias. Not very good for POS demonstrations in the dealers showrooms on cold winter days. Sorry if I am repeating myself, even this trick bias circuit is documented elsewhere on this site.

It is MOSFET output stage. I'd not actually paid much attention to it before as was given it as a stop gap until I got the active amp packs built up but someone mentioned something about the design on another forum so I went to look it up.

Right now I'm in the position that I have other things to fix in my system than the power amps.

So the TL431 is probably used a gate bias spreader and doesn't need the tempco correction.
A great application.
Low dynamic impedance is your friend.

Lateral mosfets like used in those power amps have a temperature coefficient that moves in the opposite direction from bipolar power devices as typically used. At lower class A current the bias shifts in the same direction as bipolar but above around 100mA of class A current they reverse direction and self-protect... not only on the macro level, but at micro level the mosfet power devices don't suffer from local hot spots, sharing current internally between parallel mosfet cells. This makes mosfet amp designs harder to kill and somewhat idiot proof wrt keeping the smoke inside, so they attracted a bunch of marginal designs.

The low impedance is not nothing in that context, a real problem with designing mosfet amps is driving the high gate capacitance to deliver decent current rate of change you need to be able to slew the gate drive requiring robust pre drivers. Some early mosfet amps suffered from weakness in the gate drive, and more.

JR

PS: Note class D amps typically use vertical mosfets that switch faster, and have lower gate capacitance. I wasted a bunch of bench time trying to design a power amp using vertical power fets (much cheaper)... that one didn't see daylight either.

This beasty has ridiculous slew rate for an audio amp*. It also wins an award for perhaps the worst bit of DIY construction ever. I'll take some photos when I get around to fixing its brother. But hey, two free amps, only one caught fire. Needs must etc.

Loving the power of a digital preamp though. The increase in complexity is somewhat severe tho. Previous pre was 4 fets, and input selector and a pot. But not going back now.

*I am not a believer in the audibility of slew rates that cannot exist in real music, it just came with the amp.

Yes, the same people who bring us the glorious PIC and now the ATMega. All hail Microchip.

JR: If someone wants to use this op amp to build a peak detector or condition a signal to be read by their PIC should I send you over to spank them or rain on their parade? *

I see irony.

* There seemed to be this huge assumption that every use for a rectifier or peak detector is for a damn level meter. Maybe its going to be used for signal conditioning or in a stomp box where a PIC is just silly.

You're missing the crux of the biscuit (at least of my biscuit)

No, you threw out the dough before the biscuits were even allowed to bake. I was headed somewhere with that discussion. I didn't go back there for 6 years. Not because I was pissed off but because I was working a room that just clearly wasn't primed to "get" it. The answer to where I was headed is here: https://proaudiodesignforum.com/forum/p ... 019#p12025

And it turns out the best rectifier for the job - the one that had the best waveform fidelity - was the crappiest rectifier.

Please build a meter and do the detection and averaging in DSP so we can move on.
Publish the source code or, if in assembler, the binaries so others can flash their own PICs 20 years from now.
Do a TS-2.
With that off our chests maybe we can talk about simple analog circuits and micros.
So there: I feel better now.

I promise to finish the input-capacitorless preamp.
Whomever wants to take its analog signal-conditioned output and connect it to a flying A/D can just go for it.
Whatever floats your converter...
I think I'm going to use mechanical gain control.
I've already proved I can control it digitally via SPI.

The first code I ever wrote I hand-assembled for an 8085 in the late 1970s.
This is not new to me.
There is no reason to preach to the choir.

If it wasn't for analog interfaces our computers wouldn't be very useful for us.
Not everything demands a micro or code.
Sometimes a gate, latch, op amp or comparator will do the job.